Beginner’s help - How to make a disk with perfect semicircular edges
You could think of the discs  as a doughnuts with their tops and bottoms filled in..




Alt - select the midway loops of a torus, created by with the appropriate major and minor radii
V - drag rip the edges and hover-L select the inner faces
X delete them, Reselect the boundary loops
F fill the top and bottom